1. Field of Invention
The present invention relates to a high pressure discharge lamp, and more particularly, it relates to a high pressure discharge lamp which is used as a light source for liquid crystal projectors or DLP projector apparatus.
2. Description of Related Art
Typically, a lamp unit wherein in a high pressure discharge lamp having high mercury vapor pressure, like that in Japanese Patent Application Laid-open Number H11-297268 and corresponding U.S. Pat. No. 6,271,628, is attached to a concave reflecting mirror is used as a light source for liquid-crystal projector or DLP projector apparatuses. This is because light in the visible wavelength region can be obtained at a high output level by increasing the vapor pressure of the mercury.
Recently, alternating current high pressure discharge lamps have become the mainstream for high pressure discharge lamps used in projectors, but lamps such as these have a problem in which the electrode temperature is higher than is the case with direct current high pressure discharge lamps. Alternating current high pressure discharge lamps have a higher electrode temperature because each electrode in each pair must be able to be used as a negative pole which releases thermions, thereby making it impossible to significantly reduce the size of each electrode as with the electrodes used in direct current high pressure discharge lamps. As a result, it becomes impossible to secure sufficient heat capacity to withstand operation as a positive pole.
Also, because there has been demand for more compact light sources and demand for high pressure discharge lamp designs that can withstand high power and high operating pressure as higher light intensity is required, heat capacity of the electrode head has increased and the diameter of the electrode rod in the hermetically sealed portion has decreased. However, problems occur in which a weight imbalance between the electrode heads and electrode rod diameters is thereby caused, the electrode moment at the quartz glass opening increases, equal concentric contact between the electrode rods and quartz glass inner wall is lost, and stress between tungsten and the quartz glass increases during thermal contraction. Also, educational applications are increasing in the projector market, therefore leading to increased usage aspects wherein the light source flashes more frequently and thereby requiring electrodes with higher flashing tolerance.
When such a high pressure discharge lamp for alternating current is used by repeatedly having the lamp lit at the rated power then unlit, the electrode shafts of the high pressure discharge lamp bends. As a result, a problem occurs in which the discharge arc position shifts away from the optical axis of the concave mirror and the optical output from the lamp unit decreases.
FIG. 5 is a partial frontal view showing the structure of a high pressure discharge lamp relating to the prior art wherein warping occurred in the electrode shafts. Warping of the electrode shafts herein refers to a state in which, a shown in FIG. 5, a pair of electrodes 101, 102, which are placed inside a high pressure discharge lamp 100 with the center of electrode heads 105, 106 near openings 103, 104 of the quartz glass is exposed inside the discharge space, is warped so as to separate in the longitudinal direction of electrode rods 109, 110 which are embedded in hermetically sealed portions 107, 108. The warping of the electrode shafts, in other words, the separation distance of the centers of the electrode heads 105, 106 relative to the longitudinal axis of the electrode rods 107, 108 has reached 1.5 mm or more in high pressure discharge lamps having a distance from the center of the electrode heads 105, 106 to the openings 103, 104 of the quartz glass of 5 mm. Such a degree of warping is significant enough to affect the product lifetime.
Upon inspection of the warping of the electrode shafts, it was found that, in the conventional sealing method, the quartz glass and electrode rods were always brought into contact, then sealed in order to fuse the electrode rods to the glass of the surrounding hermetically sealed tubes through contraction in a negative pressure environment. A large number of cracks were found to have occurred in locations which were contact sealed. Upon closer examination, by repeatedly having the lamp lit and unlit and thereby repeating causing thermal expansion of the electrodes, it was found that the portions wherein the cracks occurred become the fulcrum points at which the electrodes began to bend toward the direction where there were cracks or toward the direction where there were no cracks.
Even if no cracks have occurred in locations that were contact sealed to the electrode rods before the lamp was turned on, as the lamp is repeatedly lit and unlit, over time, the electrode rods and the glass will be welded together, causing cracks to form. It was found that, with these cracks as the fulcrum points, the electrode rods began to warp either in the direction cracks had occurred or in the direction where there were no cracks. In other words, the electrodes expanded and contracted repeatedly when the lamp was repeatedly turned on. It is conceivable that, because the quartz glass and the electrode rods are welded together on the side having cracks, the side with cracks became the fulcrum point at which the side that was not welded lengthens, causing the electrode rods to warp towards the side with cracks. It is also conceivable that if the welds were not released by the cracks when the lamp was turned on, then the electrode rods were re-welded due to thermal expansion and contraction when the lamp was unlit, and therefore, warped in the opposite direction to the cracks.